Two envelope soliton events below the H+ gyrofrequency with localized density depletion were discovered in low auroral region (similar to 1760 km) by Freja satellite. These events were correlated in time with the observations of the ratio of oxygen ion density to hydrogen ion density sharp increase and the electrons energization. These envelope solitons have a characteristic frequency at similar to180-190 Hz, which are obviously different from the electron-ion lower hybrid wave frequency and the helium ion gyrofrequency in low auroral plasma, but it is close to the resonance frequency of hydrogen ion-oxygen ion hybrid wave. A modulational instability model of an ion-ion hybrid wave has been discussed here. It is found that the envelope soliton below the H+ gyrofrequency in low auroral region may be generated by this modulational instability on condition that the local oxygen ion density is larger than the local hydrogen ion density.